Landslides associated to the earthquake of 7.9(Mw) of the August 15th 2007 in Peru

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Valderrama, Patricio۱; Zavala, Bilberto۱; Hermanns, Reginald L۲
Holding Date 17 September 2008

On August 15th 2007, at 23:45(UTC), a mayor earthquake of magnitude (Mw) 7.9, affected the south-center of Peru. The epicenter was located 74km offshore. The earthquake affected mainly the cities of Pisco, Chincha, Ica and Cañete (south of Lima), leaving 519 people dead and 655,674 homeless.
One of the most important geological site effects were landslides that occurred within the Andean basins and at the National Reserve of Paracas (NRP). We mapped these landslides and collected an inventory data base during a 45 days long field work along the coastal plain and the Andean zone.
The landslides triggered by those earthquakes were classified as: Rock or debris falls, topples, rock-, debris- and soilslides. These landslides affected the coastal cliff and the roads to the towns in the high part of the Andean Mountains, leaving some villages without communication for up to several weeks.
Along the coast, the most affected area was the NRP, where 15 rockslides and falls with volumes up to 535m3 occurred along the coastal bluffs and destroyed a national natural monument called La Catedral. Geologically is composed of fractured Cenozoic sandstones and shale sequences, and to a smaller portion of Paleozoic volcanic and intrusive rocks. Landslides, topples and rock falls with volumes between 10 to 150m3 affected also areas where prior to the earthquake sea lions came to rest, the whole sequels at the natural habitat of this species is still unknown. The total amount of landslides along the coastal bluffs between Cañete and Chincha was calculated to 1,215m3.
All access roads to the Andean basins were partially blocked by landslides. Geologically, this area is composed mainly for intrusive rocks (Cretaceous Coastal Batholith) and younger volcanic-sedimentary rocks. The most common landslides were rock and debris falls (individual slides had a maximum volume of 60m3). Other types of mass movements (rock, debris, and soilslides) have also been relatively small displacing in total an estimated maximum of 15,200m3 of material in all the Andean basins.
Several villages constructed on top of relict rock avalanche deposits (Laraos, Ticrapo, Tantara) were much more destroyed than nearby villages build on other deposits or on the mountain slopes. We attribute this to rearrangement and compaction of the landslide deposit material due to the shaking as clear evidence of subsidence was visible in one case. However, increased ground acceleration on top of the landslide deposits may also contributed to destructions.
Finally, the earthquake of August 15th of 2007 triggered landslides at 134 settings. These add up to a total volume of ~32,000m3, most of these in the NRP. There was no big (or destructive) landslide as in connection with other seismic events of similar magnitude, like the Ancash 1970 earthquake (North of Peru) that caused a complex slide which originated at Huascaran Mt, destroyed the city of Yungay-Ranrahirca and traveled another 150km until the Pacific ocean.